The present invention relates to a silver complex diffusion transfer process.
Theory of silver complex diffusion transfer process (hereinafter referred to as DTR process) is mentioned in U.S. Pat. No. 2,352,014 and other many patents and literatures and is well known. That is, in DTR process, silver complex salt is transferred imagewise from silver halide emulsion layer to image receiving layer by diffusion and in many cases is converted to silver image in the presence of physical development nuclei. For this purpose, imagewise exposed silver halide emulsion layer is in close contact with or is brought into close contact with an image receiving layer in the presence of a developing agent and a silver halide solvent to convert unexposed silver halide to soluble silver complex salt. The silver halide in the exposed portion of silver halide emulsion layer is developed to silver and hence is no longer dissolved and so cannot diffuse. In the unexposed portion of silver halide emulsion layer, silver halide is converted to soluble silver complex salt and is transferred to image receiving layer, where it forms silver image normally in the presence of development nuclei.
DTR process can be applied to reproduction of documents, production of lithographic printing plates, production of block copies and instant photography.
Especially for reproduction of documents and production of block copies, a negative material having a silver halide emulsion layer and a positive material having an image receiving layer containing physical development nuclei are brought into close contact with each other normally in a DTR processing solution containing a silver complex salt forming agent to form silver image on the image receiving layer of positive material.
The resulting silver image is required to have pure black color or bluish black color and besides must have sufficiently high density.
Furthermore, it is important that the silver image is high in contrast and sharpness and good in image reproducibility and besides high transfer speed is desired.
It is further necessary that good quality of positive material is not greatly depending on processing conditions (such as time and temperature).
In view of the theory of DTR process, it is easily conjectured that the process of image formation is greatly influenced by processing conditions, especially processing temperature and processing speed and this is well known in this field.
General examples of change in characteristics caused by change in processing circumstances, especially in processing temperature and running condition are enumerated below.
1. Changes in sensitivity, tone and density (reflection density and transmission density). PA0 2. Easy occurrence of stain on image receiving sheet (due to formation of fine particle silver colloid). PA0 3. Reduction of ability to form fine images such as fine lines and fine dots with increase of processing temperature or running speed.
Until now, many proposals have been made for solving these problems in respect of negative material and processing solution as disclosed in U.S. Pat. Nos. 4,302,526, 4,362,811, 4,436,805, 4,605,609, and 4,632,896, but these have not yet been satisfactory from all-round aspects. It is considered that this is because DTR process is based on the subtle balance between chemical development and dissolution, diffusion and physical development and control thereof is difficult.
In addition, the above-mentioned change of characteristics is apt to become conspicuous after continuous use of processing solution for a long time (running processing).
Generally, image forming system according to DTR process employs very simple process. For example, processor used is formed of a tray for holding transfer developer, squeeze rollers for bringing negative sheet and positive sheet into close contact with each other and a motor for rotating the squeese rollers.
In any way, it is the positive material which utilizes the silver image obtained by this simple process and the positive material must be excellent in image characteristics and physical characteristics. For example, in order for the image receiving layer of positive material having a high mechanical strength, there is a method of increasing hardness and for this purpose it was necessary to use gelatin as main portion of binder in image receiving layer.
However, even if such positive material having image receiving layer is mainly composed of gelatin, when DTR process is carried out, it is difficult to obtain silver image of high maximum density, high contrast, high fine line reproducibility and of pure black tone, and this is extremely difficult when processing temperature is low or processing solution is exhausted. It has also been found that this defect becomes severer when a negative material containing a developing agent such as hydroquinone in at least one hydrophilic colloid layer is used.